This tutorial is for those of you who used ToirtoiseGit which has the option of combining local commits before pushing them to the remote source and want to use that option in other Git clients.
To combine (merge) local commits into one before pushing them:
1. Switch to your local branch where your commits reside (actually it does NOT have to be a different branch, you can combine git commits on your local master as well)
2. I suppose you have already made 3 commits which you want to combine (merge) into one.
Screenshots were done using "SmartGit" from syntevo.